Grinding dust resulting from grinding a ferrous metal such as bearing steel, carburized steel and the like are collected in the form of cotton-like (fibrous) aggregates including: a grinding fluid containing water and oil; abrasive grains and the like. Since the cotton-like aggregates contains a large quantity of pure iron, there have been suggested a technique for reusing the aggregates as a steelmaking material. Japanese Unexamined Patent Publication No. 2001-241026, for example, discloses a technique of forming a briquette for use as steelmaking material. The briquette is formed by the steps of: forming a porous mass (a brittle compact) by compression molding the cotton-like aggregates with a press, the aggregates having adjusted contents of water and oil; impregnating an aqueous solution of solidification assistant into the porous mass by dipping the porous mass in the solution; and subjecting the porous mass to natural seasoning (curing).
The above manufacture of the briquette for use as steelmaking material requires a substantial length of drying time because the porous mass is naturally seasoned. In this connection, an attempt has been made to quickly dry the mass using a drying furnace. Unfortunately, however, the porous masses formed by compression molding are significantly varied in bulk density. When the masses are dipped in the aqueous solution of solidification assistant, therefore, the amount of water impregnated into the porous masses is also varied greatly. If the bulk density of the porous mass varies in the range of 1.5 to 3.5, for example, the water content of the porous mass impregnated with the solidification assistant varies in the range of 20 to 200 cc. Therefore, when the porous masses are dried by means of the drying furnace, it is required to set the drying time according to a porous mass having the greatest moisture content.
The just-molded porous mass is increased in temperature by 30 to 50° C. due to the compression molding. If the porous mass is immediately dipped in the solidification assistant, the temperature of the porous mass is lowered by 20 to 30° C. because the solidification assistant is at or slightly lower than normal temperatures. Accordingly, the subsequent drying step takes much time and energy to heat the porous mass to the initial temperature.
Hence, the attempt to shorten the drying time of the porous mass by using the drying furnace encounters a problem that the drying time is not effectively shortened while a large quantity of energy is consumed for drying.
In view of the foregoing, the invention has been accomplished and has an object to provide a manufacturing method of briquette for use as metal stock which is adapted for an effective reduction of the drying time of the porous mass such as to save energy for drying.